Prior art circuits based on PWM (or LC3) techniques use an electronic switch associated with an induction coil to control the current.
It has been recently been observed that although this type of current limiting circuit (or switch circuit) provides very low resistance during a normal constant-load operating state, the combination of the control device, the inductance coil, and the load capacitance gives rise to a resonant network.
This LC resonant network produces a relatively high dynamic impedance (which is unacceptable in some applications), and this may give rise to problems when more than one load is connected to the overload-limiting circuit.
The operating principle of a conventional type of PWM (or LC3) switch is illustrated in FIG. 1 and consists in using a hysteresis comparator to monitor the change in current flowing through the induction coil during current limitation. The hysteresis comparator defines maximum and minimum current levels through the induction coil, which levels are predetermined as a function of the application.
A major drawback of this solution lies in the output impedance increasing linearly with frequency (20 dB/decade) because of the presence of the induction coil required to enable the PWM (or LC3) control circuit to operate. This behavior is illustrated in FIG. 2 where the impedance of the switch device is essentially determined at low frequencies by its DC resistance RDC (which resistance is given by Np.Ron where NP is equal to the number of control circuits in parallel and Ron is equal to the resistance of each circuit).
It is easy to understand with the circuit shown in FIG. 1 that when the current through the induction coil reaches a maximum value in the current-limiting mode of operation, the above-mentioned comparator causes the switch to open, and the energy stored in the induction coil is discharged to circuit ground via a diode, and when the current flowing through the induction coil reaches a minimum value during discharge, then the comparator closes the above-mentioned switch. The appearance of the output impedance of the FIG. 1 circuit is shown in FIG. 2 as already mentioned.
An object of the present invention is thus to provide an overload-limiting circuit of the type based on the PWM (or LC3) technique which is better at satisfying the practical requirements of previously known circuits of the same type provided for the same purposes, and in particular a circuit that it is capable of reducing the output impedance in the medium frequency range (1 kHz to 10 kHz) during normal operation of the control circuit, i.e. at constant load.